This invention relates to a method of producing gas-filled hollow glass beads.
A particular use for gas-filled hollow glass spheres is as fillers for emulsion-type explosive compositions for example as set out in British Patent Specification Nos. 1 593 163 (Atlas Powder Company) and 2 055 358 (E. I. Du Pont De Nemours and Company), corresponding respectively to French Pat. Nos. 2 370 015 and 2 463 110.
For example, the Atlas Powder specification discloses a cap-sensitive explosive composition consisting of from 3 to 10% by weight of carbonaceous fuel including an emulsifier, a discontinuous aqueous oxidiser solution phase of from 10 to 25% by weight water and from 35 to 86% by weight inorganic oxidising salts composed principally of ammonium nitrate and closed cell void containing materials sufficient to yield a density of the emulsion explosion composition in the range of from about 1.1 to 1.3 g/cc. It is stated that the preferred closed cell void containing materials for use in this way are glass beads having a size in the range 10 to 175 .mu.m and a bulk density in the range 0.1 to 0.4 g/cc which can constitute 1 to 10% by weight of the explosive mixture. It is implicit in this that the glass beads may be present in amounts exceeding 40% of the total volume of the mixture. The incorporation of such glass beads in the explosive emulsion enables the formulation of a composition which is sensitive to a conventional No. 6 electric blasting cap without sensitizing it to detonation by fire, bullet, impact, friction or static electricity.
Hollow glass beads having a size and density within these ranges may readily be formed by spraying an aqueous solution or suspension of glass-formers so that droplets of the solution travel through a firing zone at glass forming temperature. Cellulation of the beads is then caused by evaporation of water within each droplet after an exterior skin has been formed by the firing. If desired a substance such as urea which will evolve gas at the firing temperature may be added to the solution. Specific examples of such a process are described in British Patent Specification No. 1 568 817 (Sovitec S. A.) and counterpart U.S. Pat. No. 4,063,916.
When glass beads are formed in this way, they have certain properties which could be improved upon for the purpose in view.
In particular, the beads, being largely formed from soluble glass-formers, are liable to attack by hydrolysis. Thus they will age when incorporated in a water-in-oil emulsion so limiting the storage life of the explosive.
Furthermore, the gas pressure within the hollow glass beads on cooling after manufacture is typically about 0.3 to 0.4 bar and this low pressure tends to act as a brake on the speed of detonation of an explosive in which they are incorporated.